1. Field of the Invention
This invention relates to valves intended to vary or modulate the flow of gaseous fuel to an atmospheric burner in response to a change in load.
2. Discussion of the Background
It is often desirable in the design of gas-fired equipment, for instance instantaneous water heaters or circulating hot water boilers, to provide a gas fuel delivery apparatus that can automatically vary the flow of gas to the burner in response to a change in load. Regarding an instantaneous water heater, for example, water flows through the heat exchanger at variable rates depending on the hot water withdrawal rate at one or more remote taps. In addition to variable flow rate, the water may enter the heater at varying temperatures depending, for instance, on the season of the year. Since the intent of the heater is to deliver hot water at a specified temperature, it follows that the burner must deliver heat at a rate proportional to the flowrate through the heat exchanger and the temperature rise from inlet to outlet that accords with the desired outlet temperature. Many instantaneous water heaters incorporate a mechanism which varies the gas flowrate to the burner in response to changes in the load placed on the heater as described above.
A similar situation with regard to varying loads can pertain to hot water circulating boilers as well. In this case, the heat exchanger is part of a circuit through which water or some other fluid is pumped. In some instances, the flowrate through the heat exchanger can vary; for instance in a zone heating circuit served by one or more pumps. Also, a change in load can be reflected in a change in the temperature rise effected in the water passing through the heat exchanger. In some boiler applications, it is desirable to run the boiler at various outlet temperatures, depending for instance on the outdoor temperature (space heating application) or domestic hot water draw (for the case where the boiler also heats domestic water either directly or indirectly through another heat exchanger).
Modulating gas valves of various types have been used with both instantaneous water heaters and with circulating hot water boilers. Many of these heaters and/or boilers employ atmospheric Bunsen-type burners, and this discussion relates to these (as opposed to so-called power burners). Various moveable means are employed in these valves; some are mechanically actuated, some are pneumatically actuated, and some are electrically actuated. Regardless of the actuating means, however, these modulating valves all share a common characteristic, which is that their purpose is to modulate the pressure of the gas immediately upstream of a fixed orifice (or orifices) that discharges the gas into the mixing tube (or tubes) of a Bunsen-type atmospheric burner. To put it another way, gas flow modulation is effected by varying the gas pressure drop through a fixed orifice that discharges into the mixing tube of the burner. This may be called the first modulating method pertaining to Bunsen-type burners.